There are various ways of compressing video information. In particular, there are three standards under which compression may be carried out: JPEG, MPEG AND H.261. These are discussed, for example, in U.S. Pat. No. 5,212,742.
Video information is commonly formatted as a series of fields. The original information which is to be converted into, and displayed in, a video format may not be immediately compatible with the field rate at which the information is to be displayed. For example, a celluloid film is shot at a rate of 24 frame/sec. (24 Hz) while, for example, the NTSC television system has a field rate of almost 60 Hz. The technique of increasing the frame rate of the film images to match that of the television system film rate is known as pulldown conversion.
Continuing with the above example of displaying a film in a NTSC standard format, a `2/3pulldown` conversion could be used in which each film frame is repeated for either two or three consecutive field periods at the video field repetition rate. The number of repetitions alternates so that the first frame is displayed twice in two consecutive field periods, the second frame is displayed three times in three consecutive field periods and so on. Thus, in one second twelve film frames at 24 Hz will each have been generated twice (i.e. for 24 field periods) while the other twelve film frames will each have been generated three times (i.e. for 36 field periods). The total (24+36) equals the 60 fields in one second at a 60 Hz field rate.
Pulldown instructions may be generated remotely and signalled to the video decoder associated with the displaying device or be generated locally at the video decoder. In the signalled pulldown, the encoder performs the pulldown calculations and signals specifically which frames are to be repeated, for example using the `repeat-first-field` flag in the MPEG-2 video syntax. The decoder simply obeys the remotely generated instructions received.
In local pulldown, the encoder encodes the film information and transmits it to the receiving device. There is no information in the transmitted signal to tell the decoder at the receiving device how to perform the appropriate pulldown conversion (e.g. the `2/3 pulldown` referred to above). The decoder must, therefore, calculate how to perform the appropriate conversion from the transmitted film frame rate to the displayed field rate.
If only pulldown conversion from the 24 Hz frame rate to a 60 Hz field rate were required, the single 2/3 pulldown conversion would be relatively easy to implement. However, other pulldown schemes are required. For example, the 24 Hz film frame rate may need to be converted to a 50 Hz field rate for the PAL television format.
Furthermore, an additional complexity in the NTSC television system is that the actual field rate is not 60 Hz but 60000/1001 Hz. Thus, the regular alternating 2/3 pulldown yields a field rate that is actually too high.